Frequency measuring and responsive circuit arrangements



Sept. 19, 1961 R. H. BRlTT FREQUENCY MEASURING AND RESPONSIVE CIRCUIT ARRANGEMENTS Filed July 31, 1958 pra/aff/'p/M/ 2 o square roof af fffyz/encj 2 Sheets-Sheet. 1

Sept. 19, 1961 R. H. BRITT 3,001,132

FREQUENCY NEASURINC AND RESPONSIVE CIRCUIT ARRANGEMENTS Filed July 31, 1958 2 Sheets-Sheet 2 This invention relates to frequency measuring and responsive circuit arrangements and has for its object to provide improved and simple means for indicating or producing a response may be present in a predetermined band of frequencies. Though not limited to its application thereto, the iuvention is primarily designed for, and is of especial advantage in, Doppler radar systems, that is to say, systems in which information is obtained -by utilizing Doppler effect due to the relative movement of a radar system and a target with respect to one another. A type of such Doppler radar system to which the invention is particularly applicable is the speed and/r drift of an aircraft is ascertainedl by transmitting radio Waves to the surface of the terrain over which the aircraft is moving and measuring the Doppler shift in the returned reflected waves to ascertain the aircrafts speed and/or drift. in this and in other types of Doppler radar system the measurement of the Doppler frequency is a matter of considerable difficulty and is commonly accomplished by measuring the frequency of a local oscillation generator which is adjusted'to correspond with the Doppler frequency. In present practice the adjustment of the local oscillation generator--A at leat the adjustment to approximate correspondence with the Doppler frequency-is effected manually. present invention may be applied with considerable advantage to enable such approximate adjustment to be done entirely automatically.

According to this invention a device for ascertaining the frequency of a signal present in a predetermined band of frequencies comprises a plurality of narrow band resonators resonant at different frequencies within said band. and between them substantially covering said band; av

leaky condenser circuit; a plurality of rectiers connected each between one side of said condenser circuitand one side of a different resonator; a plurality of output circuits each fed from a dilerent resonator; means for feeding input signals to all the resonators, a variable group selector adapted, in each position of adjustment, to counect the output circuits 1n two groups, one consisting Aof output circuits (if any) appropriate to resonators resonant at frequencies on one side of one frequency .and

the other consisting of output circuits (if any) appropriate to resonators resonant at frequencies on the other side of said one frequency, said selector leaving unconnected the output circuit appropriate to the resonator which is resonant at said one frequency;

includes that output circuit connected to that resonator adjusting said selector until output circuit.

With this arrangement voltage built up across the leaky i condenser will back oif all the rectifiers connected thereto so that, if there is an incoming signal of such fre-- cient to render all the other rectiers fion-conductive,A

from the frequency of a signal which radar that type of system inV which The.

and control means responsive to the Signal present in whichever group l60 which is predominantly responsive to the input signals for neither group includes that i -justment of said switch, to leave one unconnected and to connect together and to one contacter'- Aresponsive, to whichever 2 In these conditions only one output circuit will carry any si ate resonator by means including a non-linear element in circuit between said resonator and a point of lixed po' tential, voltage thus set up across said element being applied to said outputs circuit. A suitable non-linear element is a germanium or other semi-conductor diodev movable together and adapted, in each position of adany contacts on one side of said one contact and to con-V nect together and to the Yother contactor any contacts on.

the other side of said one contact; and control means of said contactors carries the predominant signal for producing relative movement between said contactor's and said contacts until neither contactor carries 'the predominant signal.

The control means may conveniently comprise relays 14 the control circuit of an electric motor arranged to adjust the variable group seactuating a reversing switch in lector.

The relays may be controlled each by a triggered circuit adapted to be triggered bythe prodominant signal linto a'condition in which it energizes its appropriate relay and to remain in that condition for a predetermined short period of time after each triggering. This however',l

is obviously not the only wayof controlling the relays. For example in some cases it maybe preferred forthe relays to be controlled tailed high frequency response. Owing to the pulsed nature of the inputs to these ampliers-repeated pipsthe stage of which each amplifier consists may be largely A. C. coupled, D.C. restoring clamp circuit means, 'as known per se.

ln the most important application of the invention the input signals are signals j band of frequencies and the which is to a Doppler radar, in a predetermined Doppler variable group selector is ganged with the frequency conftrol of a .variable frequency oscillator calibrated to produce, at each position of adjustment effected by said selector, a frequency producing the useful output signal.

The individual resonant frequencies of the resonators l are preferably spaced logarithmically over the band cov- Y ered thereby and preferably also each is of the same Q v value so that the bandwidths covered by the individual If this is done and the input signals are pre-amplified by an inversely proportional to the i square root of frequency the resonators will 'respond the predetermined presence of White noise only. vBy

resonators increase logarithmically with frequency.

amplifier having a gain equally in the absence of a signal'in band and in the white noise -is meantnoise of substantially the same i level over the whole band.

Thev invention is illustrated vin and further explained in; drawings, in which: l

connectionwith the accompanying FIG. l is an explanatory graphical curve;

VFIG. 2is a vdiagram of one embodiment of the invenl tion as applied to an aircraft speed or drift indicator of the Doppler radar type; and

FIG. 3 is a diagram illustrating a preferred modifica-' iv tionl of FIG. 2.

Referring to FIG. l, this problem which the present invention is Patented Sept. 19,

gnal. y Y. Preferably each output circuit is fed from the appropri-' I=KE5 where I is the current and E is the voltage across thel of said contacts each by a 'D.C. amplifier of cur- V corresponding to that of the resonator illustrates graphically thei'f designed to solve@ acca-isa In l the curve represents power per cycle P plotted as ordinates to an arbitrary scale against frequency F as abscissae. As will be seen the curve consists of a peak rising out of what may be termed a plateau. The plateau is white noise of ordinate value and may extend over a predetermined band exemplified as from 1 kc./ s. to 15 kc./s. This would be the sort of band with which an aircraft speed or drift indicating ljoppler radar system might have to deal, the Doppler frequency to be ascertained lying anywhere within this band. Arising up from the plateau is the peak which occurs at frequency Fo and is the Doppler signal whose frequency is to be measured. To give a practical example, the width of this peak at half the peak amplitude might in practice be $0.05 F0. The requirement is to adjust automatically the frequency of a local frequency generator variable over the range l-l kc./s. (for example) so as to make it correspond at least approximately with .the frequency of the signal F0.

FIG. 2 shows diagrammatically an embodiment of the present invention which will solve the above problem.

Referring to FIG. 2, the whole `band of frequencies within which the Doppler frequency Fo may lie, is derived in `any manner known per se. Since the apparatus for derivin-f' the frequency Fo is known and forms no par-t of the present invention, it is not described herein. The input signal band is applied at terminal ll to an amplifier 2 of known form and having a voltage gain inverselyproportional to the square root of the frequency.l AOutput from the amplifier 2 is fed to a drive amplifier 3 of high output impedance which in turn feeds the seriersconnecte'd primaries of a number of transformers T1, T2, T3 Tn. The number of transformers depends upon design requirements. The secondary of each transformer feeds a resonator constituted by a parallel tuned circuit C1, C2', C3 Cn. All these tuned circuits. have thevsame' Q value and their resonant frequencies are spaced logarithmically, each tuned circuit being ofa different resonant frequency and the said tuned' circuits between them covering the whole band fed in-from the amplifier Each tuned circuit is connected to a rectifier131,132,` D3 Dn and its resistance is represented by a' resistance IRI, 1K2, 1K3 lRn which may be an actual resistance or may be merely inherent resistance'. All the tuned circuits are connected through output resistances 2K1, ZRR, ZRS 2Rn to earth, and a storage condenserK shunted by a suitable resistance is connected between eatli and those .sides of all the diodes remote from their associated resonators.

The live sides of the output resistances'ZRi to 2Rn are connected as indicated to different fixed contacts L1 to L'n of a variable group selector constituted by la switch. This switch has two spaced moving arcuate c'oitactors CTIA and GT2 which contact as shown with the contacts and each of which is connected to. a xed' arcuate contacter CTS or GT4 through a brush B1 or B2. The moving. contactors can be rotated together by rotating shaftltl. It will be seen from the diagram that the moving contactors' are so arranged that in any position one of the contacts L1 to L,J is left unconnected; all said contactslf any) on one side of the unconnected contact are` connected 'to the contactor CT1; and all the remaining contacts to" Lnonlthe other side of the unconnectedcontact are` coni nected tothe contactor GT2. The fixed contactors C11?,V GT4 are connected to provide control of suitable circuits 5 and 6 for controlling the operating ,coilfs of a reversing relayswitch generally designated 7. The circuits 5i and may, for example, be bi-stable cil-c :uitsaslknownper se, in whchcase the fixed .contacts.CT3 andlCvI41arefconnested to .the triggering circuits thereof.: The circuits 5 and; 6 may', however, be D.C. Yamplifiers of curtailed high frequency, response. These amplifiers maybe, asknown per se, with largely A.C. coupled stages and provided'with D.C. restoringclamp circuits, also as known per se. The means for curtailing' high frequency may besimply aresistance-capacity circuit. Such ourtailment has the effect of increasing the duration of the pips at the expense of amplitude, thus reducing the voltage handling requirements of the amplifier and averaging the pip signal over a period of a few nulli-seconds. This averaging period is determined by the resistance-capacity time constant and may be of the same order as the operating time of the relay, or a little longer-#for example from 5 to 50 milli-seconds.

'I'he reversing relay switch 7, which supplies driving current to an electric motor 8 from a D.C. source connected at 9, is arranged in manner known per se with two operating coils. If neither` of these coils is energized the switch is open; if one of them is energized the switch connects the motor 8 to the terminals 9 with one polarity; and if the other coil is energized the switch connects the motor 8 to the terminals 9 with the other polarity. The motorS is mechanically connected to the operating shaft 10 of theV group selector switch so that rotation ofthe motorin one` direction or the other rotates the contactors CT1 and GT2 in one direction or the other. The motor 8 also drives the slider 11 of a potentiometer whose resistance 12 is connected across a D.C. source at the terminals 13. This D.C. source dri-ves through the potentiometer 11e-12 the motor portion of a phonic wheel generator 14 represented conventionally and of the well known form consisting of an electric motor driving a toothed iron wheel, the teeth of which pass through a gap in the magnetic circuit of an inductive pick-up coil. The frequency of the generator i4 is therefore directly fproportional to the speed at which the motor driven portion thereof rotates and this in turn is determined by the position of the slider 11 on the resistance 12.

The resonators with their associated rectiers are so designed that inthe presence of whitenoise alone they all respond equally. When, however, a signal such as that shown by the peak of- FIG. .l is added to the white noise, one (or possibly two) of the resonators will respond predominantly and itsl associated rectifier will feed pips of current into the condenser building up its voltage to a value at Whichall the other rectifiers are cut off and maintaining that voltage so long as the signal is present. Pips of voltage will therefore be fed to that one of the contacts L1 to Lnofthe lswitch which is connected to the resonator infquestion.V Provided that the said one contact is not the one which for the moment happens to be disconnected, the result will bethat either the contactor CT, or the contactor GT2 will carry a predominating signal, for, in the conditions stated, only that one contact has a signal at all. The predominating signal is used to cause the reversing switch to be closed in one directionl or the other .and themotor 8 to run in one direction or-the other. This s done'by the circuits 5 and 6 which, as already stated, mightfbe known bi-stable circuits of the flip-flop Vtype of D.C.' amplifiers with curtailed high frequency response. If-theyare dip-flop circuits each is arranged to be triggered by the predominating signals from a normal stable condition tothe other condition in which it energizes the appropriate relay winding of the reversingswitch, automatically returning-from said other condition after the expiration ofl a predetermined short time, e.g. halfasecond. If DtC. amplifiers are used they are employed to the relay windings of the reversing switch in obvious mannerto produce the required operational result. Since both flip-fiop Ycircuits and D.C. amplifiers are well known per se the circuits 5 and 6 are not shown in detail but are represented merely by the blocks carrying the references 5 and 6; The motor drive to the'selector switch is so arranged that'on energization of the motor 8 it drives the said selector switch until the gap between the contactors CT; and GT2 comes opposite the fixed contact which is carrying the predominating signal. In FIG. 2, in which there are twenty-five contacts shown (this by way of example only) the position of the selector switch is that which would finally be adopted if the cenj tral contact (L13) were the one carrying the-predominating signal. When the contact carrying this signal is disconnected by the selector switch, the reversing switch 7 opens circuit and the motor 8' stops. The motor 8 also drives the potentiometer 11--12 and this drive is so arranged and the generator 14 is so designed that when the motor 8 has adjusted the selector switch to that position in which the contact carrying the predominating signal is disconnected, the adjustment of the potentiometer and the speed of the generator 14 are such that the said generator produces an output frequency which corresponds to the resonant frequency of the resonator providing the said signal. Accordingly the selector switch will automatically move round until its position corresponds with the position of the frequency of a signal in the input signal band fed in at 1 and when it has done this the frequency of the generator 14 will correspond with that signal frequency and may be measured or utilized in any manner known per se.

FIG. 3 illustrates a preferred modification of the embodiment of FIG. 2. The modification consists in replacing each of the simple resistances 2R1, ZRZ, 2K3 ZRn of FIG. 2, and across which the output signals are built up, by a non-linear element-preferably a germanium or other semiconductor diode whose voltage (E) current (I) law approximates, in the early stages of forward conduction, to the equation I=KE where K is a constant. FIG. 3 shows only one of the resonators of FIG. 2- the uppermost resonator of that figuremodified by replacing the simple resistance 2R1 by a germanium diode marked 2R1 in FIG. 3. It is, of course, to be understood that, in this modification, each of the resistances ZRl, 2K2 2Rn is similarly replaced by a germanium diode but, for simplicity in drawing, FIG. 3 shows only the one circuit. Apart from the substitution of diodes for simple resistances as described, the remainder of the embodiment illustrated by FIG. 3 is as in FIG. 2.

I claim:

1. A device for ascertaining the frequency of a signal present in a predetermined band of frequencies, comprising a plurality of narrow band resonators resonant at different frequencies within a predetermined band of frequencies and between them substantially covering said band of frequencies; a leaky condenser circuit; a plurality of rectiers connected each between one side of said condenser circuit and one side of a different resonator; a plurality of output circuits each fed from a dilferent resonator; means for feeding input signals to all the resonators; a variable group selector, adapted in each position of adjustment, to connect the output circuits in two groups, one consisting of output circuits appropriate to resonators resonant at frequencies on one side of one frequency and the other consisting of output circuits appropriate to resonators resonant at frequencies on the other side of said one frequency, said selector leaving unconnected the output circuit appropriate to the resonator which is resonant at said one frequency; and control means responsive to the signals present in that group which includes that output circuit connected to that resonator which is predominately responsive to the input signals for adjusting said selector until neither group includes that output circuit.

2. A device as claimed in claim l wherein each output circuit is fed from the appropriate resonator by means including a non-linear element in circuit between said resonator and a point of xed potential, and means whereby the voltage thus set up across said element is applied to said output circuit. v

3. A device as claimed in claim l wherein each output circuit is fed from the appropriate resonator by means including a non-'linear element in circuit between said resonator and a point of xed potential, and means whereby the voltage thus set up across said element is applied to said output circuit, and wherein the non-linear element in each of the circuits are semi-conductor diodes.

4. A device as claimed in claim l wherein the variable group selector is constituted by a switch having a plurality of contacts, one connected to each of said output positions of adjustment to one contactor any contacts on one side of said one contact and to connect together, and -to the other contactor, any contacts on the other side of said one contact; and control means responsive to whichever of said contactors carries the predominant signal for producing relative movement between said contactors, and said contacts, until neither contactor carries the predominant signal.

5. A device as claimed in claim 1 wherein the control means include relays actuating a reversing switch in the control circuit of an electric motor arranged to adjust the variable group selector.

6. A device as claimed in claim 1 wherein vthe control means include relays actuating a reversing switch in the control circuit of an electric motor arranged to adjust the variable group selector, and wherein the relays are controlled each by a triggered circuit adapted to be triggered by the predominant signal into a condition in which it energizes its appropriate relay and to remain in that condition for a predetermined short period of time K after each triggering.

7. A device as claimed in claim 1 wherein the control means include relays actuating a reversing switch in Ithe control circuit of an electric motor arranged to adjust the variable group selector, and wherein the relays are controlled each by a D.C. amplifier of curtailed high .frequency response.

8. A device as claimed in claim 1 wherein the individual resonant frequencies of the resonators are spaced logarithmically over the band covered thereby.

9. A deviceV as claimed in claim l wherein all 'the resonators are of substantially the same Q value and the input signals by an amplifier having a gain inversely proportional to the square root of frequency.

l0. A Doppler radar system including a device as claimed in claim l and wherein the input signals are signals in a predetermined Doppler band of frequencies and the variable group selector is ganged with the frequency control of a variable frequency oscillator calibrated to produce, at each position of adjustment effected by said selector; a frequency corresponding to that of the resonator producing the useful output signal.

References Cited in the le of this patent UNITED STATES PATENTS 2,753,524 Newsom July 3, 1956 OTHER REFERENCES of said switch, to leave one of 'said contacts unconnected, and to connect together, and

Freystedt et al. May 23, 1939 

